SPID#: 71 This lab continues to study the primate's dorsal lateral geniculate nucleus to understand its structure and function. The dorsal lateral geniculate nucleus (dLGN) is the thalamic region responsible for transmitting retina signals to cortex. Brainstem pathways to this nucleus have been described in several species and are believed to control the retinocortical pathway depending on the state of the animal (awake, asleep, drowsy, etc.). The purpose of the studies was to (1) determine all of the subcortical sources of afferents to the dLGN in a higher primate, the macaque monkey, whose visual system is similar to that of humans, and (2) compare neurons in normal and monocularly deprived monkeys for differences in synaptic inputs. It was concluded that (1) there are seven subcortical regions that send afferents to the dLGNs of macaque monkeys; (2) the synaptic densities onto deprived neurons are higher at all dendritic distances compared to those onto normal neurons. Furthermore, HRP-filled deprived neurons received an average of 25 synapses onto their somata compared with only an average of 7 somal synapses on the HRP-filled normal neurons. Most of the increase in the number of synapses onto the deprived neurons was from GABAergic type profiles. This abnormality of the deprived neurons of the LGN could be the underlying cause of their lesser responses compared with normal or nondeprived LGN neurons. It could also be the initial stage that causes blindness in monocularly lid-sutured primates.